Scanned beam image capture devices are used for a variety of applications. In a scanned beam image capture device, a narrow beam of light is generated, often by a laser, and scanned across a field-of-view. Light from the beam is scattered, transmitted, absorbed, or reflected by features in the field-of-view according to the optical properties encountered by the beam as it scans. A portion of light from the scanned beam is “scattered” or “reflected” back to a photo-detector. The photo-detector converts the optical signal to an electrical signal. The electrical signal is used by a controller to determine one or more characteristics of the field-of-view, for example to generate an image of the field-of-view, to decode an optical indicia within the field-of-view, etc.
While terms such as “scattered” or “reflected” are used herein to describe light returned to the photo-detector, it should be understood that a variety of mechanisms may be involved in returning light to the photo-detector. The amount of light instantaneously received by the photo-detector is correlated to an instantaneous beam angle or position (which may be the same instant of time as the light being received or, for example in the case of fluorescence imaging, may be a function of one or more prior instants in time). By correlating beam position to the amount of light received by the detector, an image or other characteristic of the field-of-view may be determined.
A variety of electrical circuits, optics, and electro-optical systems have been developed for the purposes of photo detection. For many applications, small area PIN diodes are used as photo-detectors in scanned beam capture systems because of their low terminal capacitance. The terminal capacitance of the PIN diode affects the performance of the PIN diode as a photo-detector. A low terminal capacitance results in lower noise and therefore provides a higher signal-to-noise ratio. This in turn means that the low terminal capacitance PIN diode is often considered to have higher bandwidth that can be used with faster throughput. Avalanche Photo-Diodes (APDs) may also be employed as photo-detectors. However, APDs generally require higher bias voltages (˜1-2 kV) and are more expensive than PIN diodes. Relatively large area PIN diodes have been avoided in the past due to their perceived higher capacitance and resulting increased noise level.
In relatively high scan rate beam scanning systems, small area diode devices have been used as photo-detectors because they provide a low terminal capacitance. The low terminal capacitance results in a high gain-bandwidth product and a fast throughput. However, due to relatively small numerical apertures of small area diode devices, overall sensitivity of the small area diode devices may be restricted due to limited light collection. Alternatively, relatively complex optics may be used to de-scan the returned light and focus it onto a small photo-diode, but this may add cost and complexity to a product.
Embodiments of the present disclosure provide improvements over the prior art.